Transmission system



July` 5, 1949. J. GOLDSMITH 2,475,563

` TRANSMISSION SYSTEM Filed Feb. 25, 1944 dama/f 60min/UW,

Patented July 5, 1949 TRANSMISSION SYSTEM Joseph Goldsmith, Newton,Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., acorporation of Delaware Application February 25, 1944, Serial No.523,923

(Cl. 17e-44) 2 Claims.

This invention relates to a guided electromagnetic wave transmissionsystem, and more particularly to a coupling arrangement between acoaxial conductor transmission line and a dielectric wave guide.

In devices of the type to which the invention relates, a suitableterminal device is provided at the junction between the coaxial line andthe wave guide for exciting and receiving electromagnetic waves ofvarious types. The terminal may be an extension of the central conductorof the coaxial transmission line projecting from the coaxial line intothe hollow wave guide and constituting an exciting rod providing acoupling between the coaxial line and guide.

In such coupling devices a reflecting plate is provided adjacent theexciting means and it is essential that the wave reflected from thereflecting plate at the end of the guide combine in proper phase withthe direct Wave propagated from the exciting means.

I-Ieretofore it has been the common practice to space a flat reflectingplate a distance approximately equal to one-quarter of the length of thewaves on the transmission line from the exciting means. In such anarrangement the portion of the Wave guide behind the exciting meansconstitutes a resonant chamber termination for the wave guide. Suchresonant chambers add `cumbersome projections on the transmission systemwhich increase the bulk and cost thereof.

It is among the objects of the present invention to provide anarrangement in which the reilecting plate may be spaced closely to theexciting rod 'thereby avoiding the necessity for such projectingresonant chambers, but without sacrificing any of the desirableelectrical characteristics of the system.

It is a further object of the invention to provide a reflecting platewhich will introduce far less frequency sensitivity than previous typesof reflecting plates which were necessarily spaced further from thejunction between the coaxial line and Wave guide.

The foregoingVv and other objects of this invention will be made fullyapparent to those skilled in the art from a consideration of thefollowing detailed description taken in conjunction with theaccompanying drawing in which:

Fig. 1 represents a section through a coupling between a wave guide anda concentric line having a reflecting plate constructed and arranged inaccordance with the present invention; and

Fig. 2 represents a section taken along the line 2--2 of Fig. 1.

Referring to the drawing, a dielectric wave guide l, which in thisinstance is rectangular, is coupled to a coaxial conductor transmissionline comprising a central conductor 2 and an outer tubular conductor 3.The members l, 2 and 3 are of highly conductive material, such as copperor brass. One end of the tubular conductor 3 is joined to the wide sideof the wave guide l which is provided with an opening thereinsubstantially of the size of tubular conductor 3, through which openingthe conductor 2 projects for a substantial distance into the wave guideto provide an exciting rod ll, or equivalent exciting means.

Coaxial conductor transmission lines and dielectric wave guides areusually designed to carry electromagnetic waves of a predetermined wavelength in the guide, which may be represented by A. In many applicationsit is desired to transmit a range of frequencies corresponding to a waveband extending on either side of for one or two per cent. thereof, or insome instances as great as ten per cent.

When electromagnetic waves of sumciently high frequency are transmittedalong the coaxial transmission line, waves are propagated along thelength of the wave guide l in both directions from the terminal deviceprovided by the projecting end l of the central conductor 2. In orderthat the reflected waves combine in proper phase with the oppositelydirected waves, it is the common practice, as pointed out above, toprovide an end plug or plate having a flat reflecting surface whichsurface is spaced a distance approximately equal to Ml, or an oddmultiple thereof, from the exciting means. I have found thatsuch spacingmay be avoided and the electrical characteristics of the system improvedby providing the plug 5, which closes one end of the wave guide I, witha cylindrically concave surface 6 facing the terminal rod 4 to reilectthe waves propagated in the undesired direction back in the desireddirection along the length of the guide. Preferably the plug is sopositioned that a straight yline joining the ends of the plug, whichproject the farthest into the wave guide I, will pass through or closeto the axis of the exciting rod 4. The plug 5, however, should notocclude the opening of the coaxial line into the Wave guide. Suchocclusion may be avoided by making the distance L between the centerline of the exciting rod 4 and the depth of the trough, provided by thecurved surface t, equal to or greater than the radius of the coaxialline. Thus, at the point closest to the coaxial line, the plug 5 doesnot obstruct the open end of conductor 3.

The radius of curvature of the surface 6i depends upon the geometry anddimensions of the Wave guide I and conductor 9, and of course upon therelative position of the exciting means 4. In the instance shown thewave guide l is rectangular in transverse section, the exciting rod 4enters the guide through the wide dimension thereof, and the "diameterofthe conductor 3 is substanu tially less than this wide dimension ofthe guide. Under these conditions the curvature of the surface 6 issubstantially greater than the radius of the tubular conductor. Forexample, ,in ione commercially successful embodiment of the inventionwhere the internal dimension ofthe v.wide side of the wave guide was 2%and that of the narrow side was l--l, the radius of curvature of thesurface ii was'll. Inth'is instancelthediameter of the tubular conductor3 was 1,1/2". From the foregoing it will be apparent that'in practicethe axis of curvature of the cylinder 6 will vlie'wi'thin the guide I onthe opposite side of 'the'rod d from said surface. YThe axis ofcurvature will extend parallel to the axis of the 'rod 4 and will lie inthe same llongitudinal plane through the guide as the axis of said rod.

lThe axis of the curvature of the surface 6 'is in any case parallel tothe axis of the exciting rod '4. Where 'the diameter of the outer'tubular `conductor of the transmission line more `closely approachesthe dimension of the side of the guide, through which 'the exciting 'rod4 projects, the radius of curvature will become less and inthe limitinginstance approach the radius of'the conductor 3. YlE'or example, wherethe coaxial transmission line'enters the wave guide through the narrowside thereof, and the `exciting means 'eX- tends parallel to the widesides of the guide in-v stead of tothe narrow sides as shown, it will be'apparent that vthe diameter of the tubular con-I ductor 3 will moreclosely approach this narrow dimension of 'the guide and the radius 'of'curvature of the surface E will approach the 'radius of 'the tubularconductor. It will be apparentthat under "these conditions the axis ofcurvature of the surface 6 will still be made parallel `to the axis-'oftheexciting rod 4.

'I have `found that when constructed 'asdescribed in the foregoing, thereflecting vsurface Blfunctionsto reflect the waves propagated inthedirection ofthe yreflecting lsurface in phasewith `the waves `propagatedin the direction 'ofthe length of 'thefguide and without substantialv`differ-ences in 'the'attenuation of the waves through- :out thefrequency range of the system. Thus the reflecting platefintroduces farless'frequencysenysitivity than previous types of re'ecting platesispaced vat 'such 'a distance as to provide "ares'onant chamber between`the exciting 'rod Vand the '.refiectingplate.

.iilthough the reflecting surface -is actually :spaced from theexcitingmeans vby a distance sui'cie'nt-to leaVethe-'opening betweenthe coaxial.line and-wave Lgrui'de v'Ifree and unobstructed, 'the effect :of"reecting surface is the samefas farf-ars ,refiecticnless .transmission,is concerned, as Eif a flat reflecting surface were placed 'very :closeto or coincident with the exciting means. The present invention providesan effective reflecting plane very close to or coinciding with theexciting means without the practical diiiiculties which actualcoincidence by a physical surface would'introduce. Accordingly, it willbe under stood that, as used herein and in the appended claims, suchexpressions fas having an effective reflecting plane substantiallycoincident with said exciting means refer to a reflecting member whichmay not actually so :coincide but which haszthe effect of atheoretically coincident re fleeting plane.

l There has been nherein described a preferred embodiment of theinvention. Other embodiments within the scope of the. appended claimswillfbe apparent :tothose skilled in the art from a consideration of theform shown and the teachings hereof.

What is claimed is:

"1. `A coupling system comprising a coaxial conductor transmission linehaving anouter cylindrical conductor surrounding an inner conductor,'ahollow wave guide, said inner conductor extending into Said hollowwave guide through an opening :in/a `wall of said waveguide and formingan exciting rod, said outer conductor being -con- :nectedto said wall,an electromagnetic wave reflecting plug'disposed in said wave guide atone Sidefof said excitingerod, said plug having a cylindrically concavesurface adjacent said exciting rod, vthe axis of curvature of saidsurface being parallel to the axisy of said exciting rod, and the radiusof curvature of said surface being constant and at least equal to theinternal radius of said 'outer conductor fof said'transmission line,

2. A'coupling system comprising a coaxial conductor transmission linehaving an outer cylindrical-conductor surrounding an inner conductor,.fahollow wave guide,'said inner conductor extending into said hollowwaveguide through an opening in va wall of said wave guide and formingan Iexciting rod, said outer conductor being connected :to-fsad Wall, anelectromagnetic Wave reflecting :plug `disposedin said wave guide at oneside of saidnexciting rod, said plug having a cylindrically lconcavesurface adjacent said vexciting rod, the taxis of curvature ofsaid'surface being parallel Atothe 'axis of 4said exciting rod andpositioned on theside of said rod remotefrom said plug, and the radius.of .curvature o'f said surface being con- .stant'and at /leastequaltothe internal radius of said outer 'conductor of said transmissionline.

`JOSEPH.' GOLDSIVIITH.

REFERENCES CITED The following references are of record inthe (file "ofthis patent:

UNITED STATES PATENTS .Number VName Date 2,170,028 ,'Kohl Aug. 22, 19392,283,935 King May 26, 1942 :25253;501 Barrow Aug. 26, 1941 2,;.410,109Schelleng Oct. 29, 1946

